We have isolated several novel oncogenes using an expression cloning system we have developed. These genes include ECT2, OST, TIM, NET1 and NTS1 , which share common motifs for the guanine nucleotide exchange factors of the Rho family of small GTPases. The main purpose of this project is to understand the biological functions of these exchange factors.ECT2 catalyzes guanine nucleotide exchange on the small GTPases, RhoA, Rac1, and Cdc42. ECT2 is phosphorylated during G2 and M phases, and phosphorylation is required for its exchange activity. We previously showed that the ECT2 is a critical regulator of cytokinesis. In this year, we examined the expression of ECT2 in response to mitogenic stimulation. Both the ECT2 transcript and protein were induced by various mitogens in Balb/MK mouse keratinocytes in vitro. In contrast to other oncogenes whose expression is induced early in G1, ECT2 expression was induced later, coinciding with the initiation of DNA synthesis and sustained for at least 48 h. We also examined the expression of ECT2 in a liver regeneration model in mice after two-third partial hepatectomy. We found that the expression of the ECT2 transcript and protein were markedly elevated with the onset of DNA synthesis and remained elevated during G2 and M phases. The timing of ECT2 expression matched that of PCNA and partially overlapped Cdc2 expression. In situ hybridization analysis demonstrated that ECT2 was expressed at a high level in cells undergoing mitosis in regenerating liver. Moreover, expression of a dominant negative mutant of ECT2 in mouse hepatocytes showed generation of binucleate cells. ECT2 may play a role in transition of hepatocyte cellularity from binuclear to mononuclear through the regulation of cytokinesis.